(CNN)Here's some background information about Ebola, a virus with a high fatality rate that was first identified in Africa in 1976.

Facts:Ebola hemorrhagic fever is a disease caused by one of five different Ebola viruses. Four of the strains can cause severe illness in humans and animals. The fifth, Reston virus, has caused illness in some animals, but not in humans.

Ebola is extremely infectious but not extremely contagious. It is infectious, because an infinitesimally small amount can cause illness. Laboratory experiments on nonhuman primates suggest that even a single virus may be enough to trigger a fatal infection.

Ebola could be considered moderately contagious, because the virus is not transmitted through the air.

Typically, symptoms appear 8-10 days after exposure to the virus, but the incubation period can span two to 21 days.

Unprotected health care workers are susceptible to infection because of their close contact with patients during treatment.

Ebola is not transmissible if someone is asymptomatic and usually not after someone has recovered from it. However, the virus has been found in semen for up to three months, and "possibly" is transmitted from contact with that semen, according to the CDC.

Deadly human Ebola outbreaks have been confirmed in the following countries: Democratic Republic of the Congo (DRC), Gabon, South Sudan, Ivory Coast, Uganda, Republic of the Congo (ROC), Guinea, Liberia and Sierra Leone.

2014-2016 West Africa Outbreak:(Full historical timeline at bottom)Cases listed below include confirmed, probable or suspected cases of Ebola as of March 27, 2016 (World Health Organization and CDC):

Italy - 1 case, 0 deaths

Guinea - 3,811 cases, 2,543 deaths

Liberia - 10,675 cases,4,809 deaths

Mali - 8 cases, 6 deaths

Nigeria - 20 cases, 8 deaths

Senegal - 1 case, 0 deaths (infection originated in Guinea)

Sierra Leone - 14,124 cases, 3,956 deaths

Spain - 1 case, 0 deaths

United Kingdom - 1 case, 0 deaths

United States - 4 cases, 1 death (two infections originated in the United States, one in Liberia and one in Guinea)

March 25, 2014 - The CDC issues its initial announcement on an outbreak in Guinea, and reports of cases in Liberia and Sierra Leone. "In Guinea, a total of 86 suspected cases, including 59 deaths (case fatality ratio: 68.5%), had been reported as of March 24, 2014. Preliminary results from the Pasteur Institute in Lyon, France suggest Zaire ebolavirus as the causative agent."

July 2014 - Patrick Sawyer, a top government official in the Liberian Ministry of Finance, dies at a local Nigerian hospital. He is the first American to die in what officials are calling "deadliest Ebola outbreak in history."

July 2014 -Nancy Writebol, an American aid worker in Liberia, tests positive for Ebola. According to Samaritan's Purse, Writebol is infected while treating Ebola patients in Liberia.

July 26, 2014 -Kent Brantly, medical director for Samaritan Purse's Ebola Consolidated Case Management Center in Liberia, is infected with the virus. According to Samaritan's Purse, Brantly is infected while treating Ebola patients.

August 2, 2014 - A specially equipped medical plane carrying Ebola patient Dr. Kent Brantly lands at Dobbins Air Reserve Base in Marietta, Georgia. He is then driven by ambulance to Emory University Hospital in Atlanta.

August 6, 2014 - Nancy Writebol arrives at Emory in Atlanta for treatment.

August 8, 2014 - Experts at the World Health Organization declare the Ebola epidemic ravaging West Africa an international health emergency that requires a coordinated global approach, describing it as the worst outbreak in the four-decade history of tracking the disease.

August 19, 2014 - Liberia's President Ellen Johnson Sirleaf declares a nationwide curfew beginning August 20 and orders two communities to be completely quarantined, with no movement in or out of the areas.

August 21, 2014 - Dr. Kent Brantly is discharged from Emory University Hospital. It is also announced that Nancy Writebol had been released on Tuesday, August 19. The releases come after Emory staff are confident Brantly and Writebol pose "no public health threat."

September 6, 2014 - The government of Sierra Leone announces plans for a nationwide lockdown from September 19-21, in order to stop the spread of Ebola. The lockdown is being billed as a predominantly social campaign rather than a medical one, in which volunteers will go door-to-door to talk to people.

September 16, 2014 - President Barack Obama calls the efforts to combat the Ebola outbreak centered in West Africa "the largest international response in the history of the CDC." Speaking from the CDC headquarters in Atlanta, Obama adds that "faced with this outbreak, the world is looking to" the United States to lead international efforts to combat the virus. He says the United States is ready to take on that leadership role.

October 20, 2014 - Under fire in the wake of Ebola cases involving two Dallas nurses, the CDC issues updated Ebola guidelines that stress the importance of more training and supervision, and recommend that no skin be exposed when workers are wearing personal protective equipment, or PPE.

October 24, 2014 - The National Institutes of Health announces one of the Dallas nurses, Nina Pham, has been declared free of the Ebola virus. Doctors at Emory University Hospital say tests no longer detect the virus in the blood of the other nurse, Amber Vinson. Pham is released from a Maryland hospital on October 24, and Vinson is released from an Atlanta hospital on October 28.

October 24, 2014 - In response to the New York Ebola case, the governors of New York and New Jersey announce that their states were stepping up airport screening beyond federal requirements for travelers from West Africa. The new protocol mandates a quarantine for any individual, including medical personnel, who has had direct contact with individuals infected with Ebola while in Liberia, Sierra Leone, or Guinea. The policy allows the states to determine hospitalization or quarantine for up to 21 days for other travelers from affected countries.

November 5, 2014 - Nurse's aide Maria Teresa Romero Ramos, believed to be the first person to contract Ebola outside of Africa, is released from the hospital in Madrid, Spain.

November 11, 2014 - Dr. Craig Spencer, the first person to test positive for Ebola in New York City, is released from Bellevue Hospital. With Spencer free of the virus, all U.S. patients who had Ebola have recovered.

November 15, 2014 - Dr. Martin Salia, who became infected with Ebola while treating patients in Sierra Leone, arrives at Nebraska Medical Center in Omaha. Salia, a native of Sierra Leone, is a legal permanent resident of the United States married to a U.S. citizen.

November 17, 2014 - Dr. Salia dies at Nebraska Medical Center.

November 21, 2014 - The World Health Organization declares an end to the Ebola outbreak in the Democratic Republic of Congo. A total of 66 cases and 49 deaths were recorded.

December 24, 2014 - The CDC announces that a technician will be monitored for three weeks after possibly being exposed to the Ebola virus at one of the agency's Atlanta labs. The agency reports a small amount of material which may have contained the live virus had been mistakenly transferred from one lab to another.

December 2014 - American doctor Ian Crozier, who had been declared free of Ebola and released from Emory University Hospital in October 2014, finds the virus in his left eye. He had contacted the disease while working in Sierra Leone. Not at risk of spreading the disease, Dr. Crozier is treated and on his way to Liberia by early April 2015.

January 18, 2015 - Mali is declared Ebola free after no new cases in 42 days.

February 22, 2015 - Liberia reopens its land border crossings shut down during the Ebola outbreak, and Liberian President Ellen Johnson Sirleaf also lifts a nationwide curfew imposed in August to help combat the virus.

May 9, 2015 - The World Health Organization declares an end to the Ebola outbreak in Liberia. More than 4,000 died.

December 29, 2015 - The World Health Organization declares Guinea is free of Ebola after 42 days pass since the last person confirmed to have the virus was tested negative for a second time.

January 14, 2016 - A statement is released by the UN stating that "For the first time since this devastating outbreak began, all known chains of transmission of Ebola in West Africa have been stopped and no new cases have been reported since the end of November."

January 15, 2016 - A new case of Ebola - in Sierra Leone - in which the patient died - is confirmed by WHO and the CDC.

HistoricalTimeline: *Includes outbreaks resulting in more than 100 deaths or special cases.

1976 - First recognition of the EBOV disease is in Zaire (now Democratic Republic of the Congo). The outbreak has 318 reported human cases, leading to 280 deaths. An SUDV outbreak also occurs in Sudan (now South Sudan), which incurs 284 cases and 151 deaths.

2001-2002 - An EBOV outbreak occurs on the border of Gabon and Republic of the Congo (ROC), which results in 53 deaths on the Gabon side and at least 43 deaths on the Republic of the Congo side.

December 2002-April 2003 - An EBOV outbreak in Republic of the Congo results in 143 reported cases and 128 deaths.

2007 - An EBOV outbreak occurs in the Democratic Republic of the Congo (DRC), 187 of the 264 cases reported result in death. In late 2007, an outbreak in Uganda leads to 37 deaths. 149 cases were reported.

August 26, 2014 - The Ministry of Health in the Democratic Republic of the Congo notifies the World Health Organization of an Ebola outbreak in the country. It is the seventh outbreak in the country since 1976, when the virus was first identified near the Ebola river. The outbreak is not related to the ongoing outbreak in Guinea, Liberia, Nigeria and Sierra Leone.

July 31, 2015 - The CDC announces that a newly developed Ebola vaccine is "highly effective" and could help prevent its spread in the current and future outbreaks.

Photos:The Ebola epidemic

An Ebola survivor participates in a study in Monrovia, Liberia, on June 17. The country launched a five-year study to unravel the mystery of the long-term health effects that plague survivors of the viral disease. Since the epidemic started more than a year ago in a remote village in Guinea, more than 11,000 people have died, the vast majority in three West African nations, according to the latest numbers from the World Health Organization. And that number is believed to be low, since there was widespread under-reporting of cases, according to WHO.

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Women in Monrovia celebrate after the World Health Organization declared Liberia Ebola-free on May 9. Other cases have recurred since, however. Two people in Liberia have died of the disease since the end of June, just weeks after the WHO declared the nation free of the disease.

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A man walks past an Ebola awareness painting in Monrovia on March 22.

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Soldiers from the U.S. Army's 101st Airborne Division walk across the tarmac at Campbell Army Airfield before reuniting with their families at a homecoming ceremony March 21 in Fort Campbell, Kentucky. The 162 soldiers were deployed in Liberia, where they helped fight the spread of Ebola.

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Relatives weep for a loved one who it was believed died from Ebola, at a graveyard on the outskirts of Monrovia on March 11.

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Doctors Without Borders staffer Alex Eilert Paulsen watches as mattresses and bed frames burn at the Ebola Treatment Unit in Paynesville, Liberia, on January 31. The organization reduced its number of beds from 250 to 30 as gains were made in battling the virus.

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Pauline Cafferkey, a Scottish woman diagnosed with Ebola, is put on a plane in Glasgow, Scotland, on December 30, 2014. Cafferkey, a 39-year-old nurse who volunteered in Sierra Leone, was being transported to London for treatment.

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A child who survived the Ebola virus is fed by another survivor at a treatment center on the outskirts of Freetown, Sierra Leone, on November 11, 2014.

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Health workers in Monrovia cover the body of a man suspected of dying from the Ebola virus on October 31, 2014.

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Kaci Hickox leaves her home in Fort Kent, Maine, to take a bike ride with her boyfriend on October 30, 2014. Hickox, a nurse, recently returned to the United States from West Africa, where she treated Ebola victims. State authorities wanted her to avoid public places for 21 days -- the virus' incubation period. But Hickox, who twice tested negative for Ebola, said she would defy efforts to keep her quarantined at home.

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Health officials in Nairobi, Kenya, prepare to screen passengers arriving at the Jomo Kenyatta International Airport on October 28, 2014.

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U.S. President Barack Obama hugs Ebola survivor Nina Pham in the Oval Office of the White House on October 24, 2014. Pham, one of two Dallas nurses diagnosed with the virus, was declared Ebola-free after being treated at a hospital in Bethesda, Maryland. The other nurse, Amber Vinson (not pictured), was treated in Atlanta and also declared Ebola-free.

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Health workers in Port Loko, Sierra Leone, transport the body of a person who is suspected to have died of Ebola on October 21, 2014.

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Health workers bury a body on the outskirts of Monrovia on October 20, 2014.

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Garteh Korkoryah, center, is comforted during a memorial service for her son, Thomas Eric Duncan, on October 18, 2014, in Salisbury, North Carolina. Duncan, a 42-year-old Liberian citizen, died October 8 in a Dallas hospital. He was in the country to visit his son and his son's mother, and he was the first person in the United States to be diagnosed with Ebola.

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Boys run from blowing dust as a U.S. military aircraft leaves the construction site of an Ebola treatment center in Tubmanburg, Liberia, on October 15, 2014.

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Aid workers from the Liberian Medical Renaissance League stage an Ebola awareness event October 15, 2014, in Monrovia. The group performs street dramas throughout Monrovia to educate the public on Ebola symptoms and how to handle people who are infected with the virus.

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Ebola survivors prepare to leave a Doctors Without Borders treatment center after recovering from the virus in Paynesville, Liberia, on October 12, 2014.

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A man dressed in protective clothing treats the front porch of a Dallas apartment on October 12, 2014. The apartment is home to one of the two nurses who were diagnosed with Ebola after treating Thomas Eric Duncan, a Liberian national who traveled to Dallas and later died from the virus.

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A woman crawls toward the body of her sister as a burial team takes her away for cremation October 10, 2014, in Monrovia. The sister had died from Ebola earlier in the morning while trying to walk to a treatment center, according to her relatives.

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A man digs a grave on October 7, 2014, outside an Ebola treatment center near Gbarnga, Liberia.

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A person peeks out from the Dallas apartment where Thomas Eric Duncan, the first person diagnosed with the Ebola virus in the United States, was staying on October 3, 2014.

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A girl cries as community activists approach her outside her Monrovia home on October 2, 2014, a day after her mother was taken to an Ebola ward.

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A health official uses a thermometer September 29, 2014, to screen a Ukrainian crew member on the deck of a cargo ship at the Apapa port in Lagos, Nigeria.

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Workers move a building into place as part of a new Ebola treatment center in Monrovia on September 28, 2014.

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Medics load an Ebola patient onto a plane at Sierra Leone's Freetown-Lungi International Airport on September 22, 2014.

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A few people are seen in Freetown during a three-day nationwide lockdown on September 21, 2014. In an attempt to curb the spread of the Ebola virus, people in Sierra Leone were told to stay in their homes.

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Supplies wait to be loaded onto an aircraft at New York's John F. Kennedy International Airport on September 20, 2014. It was the largest single shipment of aid to the Ebola zone to date, and it was coordinated by the Clinton Global Initiative and other U.S. aid organizations.

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A child stops on a Monrovia street September 12, 2014, to look at a man who is suspected of suffering from Ebola.

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After an Ebola case was confirmed in Senegal, people load cars with household items as they prepare to cross into Guinea from the border town of Diaobe, Senegal, on September 3, 2014.

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A health worker wearing a protective suit conducts an Ebola prevention drill at the port in Monrovia on August 29, 2014.

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A burial team from the Liberian Ministry of Health unloads bodies of Ebola victims onto a funeral pyre at a crematorium in Marshall, Liberia, on August 22, 2014.

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Dr. Kent Brantly leaves Emory University Hospital on August 21, 2014, after being declared no longer infectious from the Ebola virus. Brantly was one of two American missionaries brought to Emory for treatment of the deadly virus.

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An Ebola Task Force soldier beats a local resident while enforcing a quarantine on the West Point slum on August 20, 2014.

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Local residents gather around a very sick Saah Exco, 10, in a back alley of the West Point slum on August 19, 2014. The boy was one of the patients that was pulled out of a holding center for suspected Ebola patients after the facility was overrun and closed by a mob on August 16. A local clinic then refused to treat Saah, according to residents, because of the danger of infection. Although he was never tested for Ebola, Saah's mother and brother died in the holding center.

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Workers prepare the new Ebola treatment center on August 17, 2014.

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Liberian police depart after firing shots in the air while trying to protect an Ebola burial team in the West Point slum of Monrovia on August 16, 2014. A crowd of several hundred local residents reportedly drove away the burial team and their police escort. The mob then forced open an Ebola isolation ward and took patients out, saying the Ebola epidemic is a hoax.

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A health worker disinfects a corpse after a man died in a classroom being used as an Ebola isolation ward August 15, 2014, in Monrovia.

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Aid worker Nancy Writebol, wearing a protective suit, gets wheeled on a gurney into Emory University Hospital in Atlanta on August 5, 2014. A medical plane flew Writebol from Liberia to the United States after she and her colleague Dr. Kent Brantly were infected with the Ebola virus in the West African country.

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Members of Doctors Without Borders adjust tents in the isolation area in Kailahun on July 20, 2014.

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Boots dry in the Ebola treatment center in Kailahun on July 20, 2014.

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Dr. Jose Rovira of the World Health Organization takes a swab from a suspected Ebola victim in Pendembu, Sierra Leone, on July 18, 2014.

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Red Cross volunteers disinfect each other with chlorine after removing the body of an Ebola victim from a house in Pendembu on July 18, 2014.

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A scientist separates blood cells from plasma cells to isolate any Ebola RNA and test for the virus April 3, 2014, at the European Mobile Laboratory in Gueckedou, Guinea.

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Health specialists work March 31, 2014, at an isolation ward for patients at the facility in southern Guinea.